The evolution of the third generation mobile communication systems includes higher data rates and packet oriented modes. High Speed Downlink Packet Access (HSDPA) is a new feature in WCDMA that improves throughput in the system and increases the maximum data rate for a single user. HSDPA is a packet data transmission system, where the base station schedules and transmits data packets to different Mobile Stations (MSs).
An important component to prevent from losing data packets in HSDPA is the Hybrid Automatic Repeat Request (Hybrid ARQ). The data packets are preceded by indicators that inform the receiving MS about transmission time and other characteristics of the transmission. For each packet that the MS receives, the MS transmits an acknowledgement (ACK) signal if the packet has been received correctly and a negative acknowledgement (NACK) signal if the packet was received but not correctly. It may happen that the MS does not detect an indicator signal from the base station. In that case the MS is not able to receive the data packet, and will apply discontinuous transmission (DTX), i.e. the MS will not transmit any signal at all. In other words, the MS only transmits ACK/NACK signals when it has received a packet, otherwise there is no signal transmitted.
The transmitted signals, ACK and NACK, are antipodal signals. In parallel with the ACK/NACK signal there is a pilot signal that can be used for channel estimation. The network specifies power offsets ΔPa and ΔPn for ACK/NACK transmissions, respectively. These power offsets are relative to the power for the pilot signal. The channel estimation and the known power offsets give the estimated received signal powers for ACK and NACK in case there was an ACK or NACK transmission. Thus the received signal powers and amplitudes of ACK and NACK transmissions can be estimated by the receiver.
When the base station tries to receive the ACK/NACK, there are three different possibilities: ACK, NACK, or DTX has been transmitted. Table 1 lists the target performance requirements on the physical layer that have been outlined in the 3GPP for the ACK/NACK detection. The requirements are given in the form of probabilities of erroneous detection that the physical layer should not exceed. The ability of the physical layer to fulfill the requirements depends on the ACK/NACK detector as well as the power offsets ΔPa and ΔPn for ACK and NACK transmission, respectively, which are specified by the network. In table 1, the notation P(DTX|ACK) represents the probability of detecting DTX signal when ACK signal is transmitted. It is similar for P(ACK|NACK) and P(ACK|DTX).
TABLE 1Target performance requirements on ACK/NACK receptionConditional probability:P(Detected|Propagation channelTransmitted)Case1/Case2Case3P(NACK or DTX |≦10−2ACK)P(ACK | NACK)≦10−4≦10−3P(ACK | DTX)≦10−2≦10−1
Two ACK/NACK detectors have been proposed, the Constant False Alarm Rate (CFAR) detector, and a dynamic threshold detector. A CFAR detector was presented in “Simulation conditions for HS-DPCCH (ACK/NACK) detection performance,” R4-030928, 3GPP, November 2003, and “Energy requirements for UL ACK/NACK signaling under different sets of constraints”, R1-02-0420, 3GPP, February 2002. The dynamic threshold detector was presented in “On the decision threshold for detecting ACK/NACK messages,” R1-02-0823, 3GPP, May 2002.
The CFAR detector ensures a constant erroneous detection probability of ACK and NACK when no signal has been transmitted i.e. DTX condition, regardless of the noise power. This is achieved by having an adaptive detection threshold proportional to the noise standard deviation. For a CFAR detector of ACK/NACK, two such adaptive thresholds are needed: Ta, which is negative, between “ACK” and “DTX”, in case of negative signs for ACK and Tn, which is positive, between “DTX” and “NACK”, in case of positive signs for NACK. It is shown in FIG. 1a. A decision variable z can be obtained by means of accumulating outputs of Rake combiner. If a decision variable z is less than the threshold Ta, the decision is ACK; if a decision variable is greater than threshold Tn, the decision is NACK; if a decision variable is between the two thresholds, Ta and Tn, the decision is DTX.
The threshold Ta is formed by multiplying the noise standard deviation after despreading σw with the norm of column vector given by channel estimates, ∥ĥ∥, and the coefficient α that is set to fulfill the requirement on P(ACK|DTX), while Tn=−Ta. The column vector of channel estimates is the set of weights used in a maximum ratio combiner. Other sets of weights are also possible, e.g. weights corresponding to equal gain combining. Ta is given byTa=−α0σw∥ĥ∥  (1)where P(ACK|DTX) is 0.01 for α=1.65.
The channel is here defined as the product of the amplitude of the transmitted pilot signal, i.e. the square root of the pilot transmission power and the complex-valued radio channel. In a fading channel, the CFAR detector can result in an unnecessarily high ACK power requirement, in order to ensure that the ACK signal is sufficiently above the average noise power. Such high required ACK power can be reduced by a dynamic threshold detector proposed by Philips, using information about the instantaneous propagation channel conditions, which are reflected in the estimated signal amplitude.
The dynamic threshold detector has a threshold Ta that is proportional to the product of the noise standard deviation and the estimated signal amplitude after Rake combining √{square root over (ΔPa)}∥ĥ∥2:
                              T          a                =                              -            α                    ⁢                                          ⁢                      σ            w                    ⁢                                    Δ              ⁢                                                          ⁢                              P                a                                              ⁢                                                                  h                ^                                                    2                                              (        2        )            
The constant α is selected such that in average P(DTX|ACK) fulfils the target performance requirement. The value of α depends on the propagation channel, speed, antenna diversity, as well as ΔPa.
Analogously, the detector has a threshold Tn that is proportional to the product of the noise standard deviation and the estimated signal amplitude after Rake combining:
                              T          n                =                  α          ⁢                                          ⁢                      σ            w                    ⁢                                    Δ              ⁢                                                          ⁢                              P                n                                              ⁢                                                                  h                ^                                                    2                                              (        3        )            
For both the CFAR detector and the dynamic threshold detector, in case of positive signs for ACK and negative signs for NACK, threshold Ta is positive and threshold Tn is negative. If a decision variable z is less than the threshold Tn, the decision is NACK; if a decision variable is greater than threshold Ta, the decision is ACK; if a decision variable is between the two thresholds, Tn and Ta, the decision is DTX. This is shown in FIG. 1b. 
There are several drawbacks with this dynamic threshold detector:
1) It is impossible to calculate α for all possible channels, speeds, etc.
2) The required power for NACK can become higher than the required power for ACK. The reason is that the required P(ACK|NACK) is much lower than the required P(NACK|ACK) because if NACK is detected as ACK, the physical layer will not detect an erroneous packet, and instead, the detection will be made in higher layers and thus cause longer delays for the retransmission. If the threshold between DTX and ACK, Ta, is close to the origin, the NACK power must be high to keep P(ACK|NACK) below the tight requirement.
3) The threshold depends on the product of the noise standard deviation and the estimated signal amplitude. This implies that the output of the detector depends not only on the ratio
            Δ      ⁢                          ⁢              P        a              ⁢                                      h          ^                            2        /          σ      w      but on the absolute values of
            Δ      ⁢                          ⁢              P        a              ⁢                          h        ^                    2  and σw.
The ACK/NACK signals are important in an ARQ scheme. To achieve a reliable ACK/NACK detection in HSDPA the transmission power is typically relatively high. If the ACK/NACK signals need to be repeated to achieve sufficient detection performance, the interference increases and the maximum data rate is decreased. An efficient detector is needed to minimize the required transmission power for ACK and NACK signals and to maximize the data rate.